Oil-hydraulic vehicles have long been available. An oil-hydraulic vehicle currently in use comprises an engine, a transmission, an oil-hydraulic pump, and oil-hydraulic motors and transforms the power of the engine into wheel-driving force as follows.
First, the turning force of the engine is transformed by the transmission. Second, the transformed turning force is transformed into oil-hydraulic force by the oil-hydraulic pump. Then, the oil-hydraulic force is transformed into turning force by the oil-hydraulic motors to drive wheels. Thus, the turning force of the engine is transformed into wheel-driving force.
To change gears requires a clutch which connects the crankshaft of the engine and the input shaft of the transmission; accordingly, the drive line from the engine to the oil-hydraulic pump becomes complex.
On the other hand, disclosed in Japanese Patent No. 3415824 is an art of switching wheel-driving power by using flow-control valves alone. The advantage of the flow-control valves is that driving force can be adjusted without using a clutch and a complex transmission and, hence, vehicle weight can be reduced.
It is desirable, however, if the oil-hydraulic vehicle is provided with a mechanism capable of finer control of wheel-driving force than the control by the above flow-control valves, because the oil-hydraulic vehicle can be run more smoothly and more comfortably.